We've learned John Travolta will be the first witness called today in the extortion trial over his son's death -- and after John steps down from the stand, there will be a legal dogfight in the courtroom over what could be the smoking gun in the case.

Sources say Travolta will testify for less than two hours. When he finishes, we're told defense lawyers will mount an objection to the introduction of a secret videotape which allegedly shows the attempted extortion played out.

The video was shot by authorities in the Bahamas. It shows Travolta's lawyers listening attentively as the alleged $25 million extortion demand is laid out on the table.

Story photo: Travolta recounts son's death at Bahamas trial U.S. actor John Travolta and wife Kelly Preston leave the court building in Nassau, Bahamas, Wednesday, Sept. 23, 2009. Travolta described the moments before his son's death in the Bahamas as he testified Wednesday against two people accused of trying to blackmail him with private information about the rescue effort. (AP Photo/Kris Ingraham)Associated Press

NASSAU, Bahamas -- John Travolta said Wednesday he tried desperately to save the life of his seizure-prone son and made perhaps his first public description of the boy as autistic as he testified in the trial of two people accused of trying to blackmail him.

With his wife, Kelly Preston, looking on inside the Nassau courtroom, Travolta said that he performed CPR on his son after a nanny alerted him the teen had fallen ill at a family vacation home on Grand Bahama island. Jett Travolta later died from a seizure.

"I ran downstairs with my wife to help my son," Travolta said.

Travolta is the star witness at the trial of two people -- paramedic Tarino Lightbourne and former Bahamian Sen. Pleasant Bridgewater -- who are accused of trying to extort $25 million from the movie star. Both defendants pleaded innocent to extortion charges at the trial that began Monday in this island chain off the Florida coast.

Travolta testified that he and Preston were awakened by a nanny around 10:15 a.m. on Jan. 2, the day of Jett's death. Travolta said when he went downstairs, another caretaker was doing chest compressions and he began administering breathing help.

Travolta also said -- apparently for the first time in public -- that 16-year-old Jett was autistic, confirming speculation that had swirled for years. The developmental disorder is frequently accompanied by seizures.

"He was autistic. He suffered from a seizure disorder," Travolta told the jury when asked about his son's condition.

The actor said Jett suffered seizures every five to 10 days. He said the seizures would last 45 seconds to a minute and Jett typically slept for 12 hours after each one.

The judge dismissed Travolta on Wednesday afternoon but said he would be called back later for further testimony.

Travolta, 55, has kept a low profile since Jett's death. He skipped promotional events this summer for his latest film, "The Taking of Pelham 1 2 3." He thanked his co-stars on his Web site for giving him time to grieve.

Travolta arrived at the Supreme Court building in an entourage of sport utility vehicles for his first appearance at the trial. Security was noticeably heavier than previous days, with Bahamian police officers surrounding the courthouse.

Police say the alleged scheme involved a document related to Jett's treatment. It would have released emergency responders from liability if the family refused an ambulance but police said the document did not come into play.

After waiting half an hour for the ambulance to arrive, Travolta testified, he signed the release because he initially wanted his son taken to the airport for a flight to Florida instead of a local hospital.

He did not address why Jett was ultimately taken to a hospital. But an employee of the resort community where the Travoltas have their Bahamas home, Nathan Moody, testified earlier that he looked into using two private planes to fly Jett out but neither was an option on such short notice.

Secret police videos have captured the alleged extortionists in the John Travolta case in deep conflict ... with one defendant accusing the other of trying to settle way too cheap -- this according to law enforcement sources.

As we first reported yesterday, authorities have at least two videos of paramedic Tarino Lightbourne and attorney Pleasant Bridgewater demanding millions of dollars from John Travolta's lawyers, in return for not making a document public.

Our sources say Bridgewater is caught on tape demanding many millions of dollars, but another video shows Lightbourne outraged that Bridgewater demanded what he considered such a paltry sum. His demand was exponentially higher than hers, law enforcement sources tell TMZ.

We're told the videos show them haggling over money -- and although it's been reported the pair allegedly demanded $25 million, we're told they finally agreed to $15 mil.

We're also told lawyers for Bridgewater and Lightbourne will make a motion today to suppress the videos, on grounds they were illegally recorded.

And there's a sticky problem -- we're told one of the key Bahamian cops in the video sting was seriously injured "under suspicious circumstances" and is unable to testify.

Travolta has not finished testifying. We're told he has left the Bahamas and will return next week to complete his testimony.